Esters that are derived from carboxylic acids and aliphatic alcohols and which are manufactured as residue products comprise an important and broadly used class of compounds. This invention relates to an improved method for manufacture of those compounds.
Commercial methods for manufacture of residue product esters generally use a catalyst to improve the rate of reaction and the degree of conversion of acids to ester. In most instances these catalysts must be removed from the crude ester product so that it may be refined to give a good quality ester for use in applications requiring high purity. Sulfuric acid is the most widely used catalyst for ester manufacture. It is fast and inexpensive, but has disadvantages of yield and quality, and must be neutralized with caustic and washed from the crude ester. Elimination of washing steps and providing generally more convenient methods of refining, as well as fast reaction rates, higher yields and better quality are highly desirable objectives in the esterification industry and are the concern of much investigation and patent art.
The Werber et al patent, U.S. Pat. No. 3,056,818 of 1962 disclosed the use of organotitanates as catalysts to give high yields, fast rates and good quality esters. This specification also states that the esters need not be washed and that the titanium catalysts need not be removed from the residue product esters. These catalysts, as for example, tetraisopropyl titanate and tetrabutyl titanate, although requiring higher temperatures than the traditional acid catalysts, do provide advantages of a very high conversion rate of acids to ester, while minimizing side reactions which cause color buildup in products and deterioration in the excess alcohols that may be recovered and recycled. It is clear, however, that the titanium catalyst residues, like acid catalyst residues, must be removed from the crude products. Chilton (U.S. Pat. No. 3,818,071), McGee (U.S. Pat. No. 3,717,672), Ghanayem (U.S. Pat. No. 4,007,218), and Sagara (U.S. Pat. No. 4,284,793) among others have pointed to the problems associated with the titanium residues and have disclosed methods to deal with the problems.
The method conventionally used to remove the titanium residues is the same as with acids; that is, washing with aqueous caustic. Most of the other methods proposed also involve treatments with strongly basic materials such as lime (see McGee) or solid alkali (see Sagara), although investigators since Werber have looked for more convenient methods.
An object of this invention is to provide a method for manufacture of esters that does not require caustic wash or separate treatment with strongly basic materials.
The most convenient method for removing the titanium has long been thought to be simple hydrolysis with steam to yield titanium oxides that would be insoluble and removed by filtration. Since the conventional method for removing excess alcohol from an esterification reaction mixture is by steam distillation, the hydrolysis and removal of alcohol could be done at the same time and the process would be simplified to reaction, stripping and filtration.
It is an object of this invention to provide a method wherein refining of the crude ester mixture may be accomplished by steam distillation and subsequent removal of hydrolyzed catalyst, thereby by reducing the process to its simplest form of reaction, stripping and filtration.
Previously it was not practical to refine titanium catalyzed esters in this manner, as the hydrolysis of the titanium catalyst residues during steam distillation varied markedly from ester to ester, and in all cases was incomplete. The consequences of slow and incomplete hydrolysis of the residual titanate catalyst often are severe and include an increase in acid number, retention of color, poor filtration and continued precipitation of titanium insolubles in storage, as well as poor thermal stability and poor electricals.
The reason for this incomplete hydrolysis of the titanium is not clear. Simple tetra-alkyl titanates, as initially added to the reaction mixture, are quite reactive with water and easily hydrolized to insoluble oxides upon treatment with steam at these ele- vated temperatures. It is likely that the catalyst residues are not the same structures added in the beginning of the reaction, but are larger, more complicated species, less active as catalysts and less easily hydrolyzed. Since the titanates are tetrafunctional and have the added capability of forming coordination complexes to give an effective valence of six, and since they are reactive not only with alcohols and water, but with carboxylic acids, it is not difficult to envision the formation of relatively complex and hindered species, especially in those instances where the acid component of the system is multifunctional. This might explain not only the difficulty in hydrolyzing all of the titanium in the residue products, but also the vastly differing rate and degree of hydrolysis from one instance to another.
It is the object of this invention to provide an improved method for manufacture of esters wherein the process is simplified to the three steps of refining, stripping and filtration.